There are many physiological and pathological conditions of animal tissue where the supply of exogenous ribonucleosides may have useful therapeutic applications. In a number of physiological and pathological conditions, the administration to an animal of RNA, nucleotides, or individual or mixtures of nucleosides, has been shown to improve the natural repair processes of the affected cells.
There are many important metabolic reactions that are usually functionally subsaturated and limited by availability of either substrates or cofactors. Such rate-limiting compounds may be either nutritionally essential or synthesized de novo in the body. Under conditions of tissue trauma, infection or adaptation to physiological demand, particularly when cellular repair or regeneration processes are activated, the optimum nutritional, biochemical, or hormonal environment for promoting such repair may be quite different from the requirements for normal cell and tissue function. In such cases, therapeutic benefit may be derived by providing appropriate conditionally essential nutrients, such as ribonucleosides or metabolites which may be required in quantities not usually available from a normal diet. The therapeutic potential for this strategy of directly supporting the metabolic function of damaged or diseased tissues has not been realized in contemporary medical practice.
At the cellular level of organization, there are specific metabolic responses to trauma that are involved, in a variety of tissues, in the processes of tissue repair, regeneration, or adaptation to altered functional demand. Most processes of tissue damage and repair are accompanied by a substantial increase in the activity of the hexose monophosphate pathway of glucose metabolism.
The hexose monophosphate pathway is the route of formation for the pentose sugars (e.g., ribose) which are necessary for nucleotide and nucleic acid synthesis. The availability of ribose is rate limiting for nucleotide synthesis under most physiological or pathological conditions. Rapid production of nucleotides for the synthesis of nucleic acids and nucleotide-derived cofactors (such as cytidine di-phosphocholine (CDP choline) or uridine di-phosphoglucose (UDPG)) is essential for the processes of tissue repair and cellular proliferation. Even though nucleotides are synthesized de novo from simpler nutrients, so that there is not an absolute dietary requirement for direct nucleotide precursors, many tissues may not have optimal capacity for nucleotide synthesis particularly during tissue repair or cellular proliferation.
It is possible to bypass the limited capacity of the hexose monophosphate pathway by providing preformed ribonucleosides directly to tissues where they are incorporated in the nucleotide pools via the "salvage" pathways of nucleotide synthesis. It is also possible that pyrimidine ribonucleosides may exert therapeutic influences through mechanisms unrelated to the support of nucleotide biosynthesis.
The effects of the administration of pyrimidine nucleosides, and in particular, uridine and cytidine, on a variety of physiological and pathological conditions in experimental animals, isolated tissues, and to some extent, in humans, have been extensively studied. These are summarized below.
(1) Heart
In isolated rat hearts subjected to low-flow ischemia, reperfusion with uridine induced restoration of myocardial ATP levels, total adenine nucleotide content, uridine nucleotide levels, and glycogen content. Ischemia was reported to produce a breakdown of creatinine phosphate, ATP, uridine nucleotides and glycogen. Aussedat, J., Cardiovasc. Res. 17:145-151 (1983).
In a related study, perfusion of isolated rat hearts with uridine resulted in a concentration-dependent elevation of myocardial uracil nucleotide content. Following low-flow ischemia, the rate of incorporation of uridine was increased twofold. Aussedat, J., et al., Mol. Physiol. 6:247-256 (1984).
In another study, isoproterenol was administered to rats which depleted cardiac glycogen stores and reduced myocardial UTP and UDP-glucose levels. Despite the spontaneous restoration of myocardial UTP levels, UDP-glucose concentrations remained depressed unless uridine or ribose were administered. Prolonged intravenous infusion of ribose or uridine resulted in a restoration of myocardial glycogen. Thus, there may be compartmentation of uridine nucleotides in the heart, with the pools being fed differentially by the salvage or de novo pathways of pyrimidine synthesis. Aussedat, J., et al., J. Physiol. 78:331-336 (1982).
The effects of nucleosides on acute left ventricular failure in isolated dog heart was studies by Buckley, N. M., et al., Circ. Res 7:847-867 (1959). Left ventricular failure was induced in isolated dog hearts by increasing aorta pressure. In this model, guanosine, inosine, uridine and thymidine were found to be positive inotropic agents, while cytidine and adenosine were negatively inotropic.
Sodium uridine monophosphate (UMP) and potassium orotate were found to increase the animal's resistance to subsequent adrenaline-induced myocardial necrosis. These compounds reduced mortality and improved myocardial function as assessed by ECG readings, biochemical findings, and relative heart weight. Intravenous administration of UMP exerted a more pronounced prophylactic effect than did potassium orotate. Kuznetsova, L. V., et al., Farmakol.-Toksikol 2:170-173 (1981).
In a study on the effects of hypoxia in isolated rabbit hearts, myocardial performance declined while glucose uptake with glycolysis, glycogenolysis and breakdown of adenine nucleotides were reportedly increased. Administration of uridine increased myocardial performance, glucose uptake and glycolysis and also diminished the disappearance of glycogen and adenine nucleotides from hypoxic hearts. Uridine also increased glucose uptake, glycolysis, levels of ATP and glycogen, as well as myocardial performance in propranolol-treated hearts. Kypson, J., et al., J. Mol. Cell. Cardiol. 10:545-565 (1978).
In a study of pyrimidine nucleotide synthesis from exogenous cytidine in the isolated rat heart, myocardial cytosine nucleotide levels were significantly increased by a 30 minute supply of cytidine. Most of the cytidine was recovered as part of cytosine nucleotides and uracil nucleotides. Very little of the cytidine that was taken up was converted into uridine nucleotides. These results suggest that the uptake of cytidine can play an important part in myocardial cytosine nucleotide metabolism. Lortet, S., et al., Basic Res. Cardiol. 81:303-310 (1986).
In another study, myocardial fatigue was produced by repeated, brief ligations of the ascending aorta. Intravenous administration of a mixture of uridine and inosine after the fifth such ligation temporarily stopped the development of fatigue in the myocardium. Pretreatment with an undisclosed amount of uridine prevented the decrease in maximal pressure upon aortic ligation that is observed 2 hours after aortic stenosis. Meerson, F. C., In: Tr. Vseross. S'ezda Ter., Myasnikov, A. L. (ed.), Meditsina (publisher), Moscow, p. 27-32 (1966).
In another study, the use of glucose and uridine to control contractability and extensibility disturbances in the non-ischematized compartments of the heart after myocardial infarction were studied. The deficits in contractability and extensibility were reported to be due to sustained sympathetic nervous activity. The addition of glucose or uridine in vitro restored contractability and extensibility of the isolated atrial tissue. Meerson, F. Z., et al., Kardiologiya 25:91-93 (1985).
Despite the above results which were observed in isolated hearts or in situ organ preparations, the administration of uridine to intact (i.e., alive and free-running) animals has not been demonstrated to be beneficial. Thus, while Eliseev, V. V., et al., Khim-Farm. Zh. 19:694-696 (1985) (CA 103:82603k) disclose that uridine-5'-monophosphate has a protective effect on rats with adrenaline-induced myocardial dystrophy, uridine was found to be relatively ineffective. Moreover, Williams, J. F., et al., Aust. N.Z. J. Med. 6:Supp. 2, 60-71 (1976), disclose that with rats developing hypertrophy of the heart, there was no difference between rats which were treated with uridine compared with controls. Thus, except for rats which received continuous infusion of uridine (Aussedat et al., supra), no beneficial effect on pathology related to the heart has been demonstrated with uridine administration.
(2) Muscles
Exposure to uridine has also been found to enhance glucose uptake and glycogen synthesis in isolated skeletal and cardiac muscle. Kypson, J., et al., Bioch. Pharmacol. 26:1585-1591 (1977). Uridine and inosine were found to stimulate glucose uptake in isolated rat diaphragm muscle. However, only uridine increased glycogen synthesis. Both nucleosides inhibited lipolysis in adipose tissue. Kypson, J., et al., J. Pharm. Exp. Ther. 199:565-574 (1976).
(3) Liver
Administration of cytidine and uridine has also been reported to be effective in enhancing the regeneration of the liver in rats acutely poisoned with carbon tetrachloride. Bushma, M. I., et al., Bull. Exp. Biol. Med. 88:1480-1483 (1980).
There have been a number of reports relating to the therapeutic administration of nucleotides and RNA. The beneficial effects of RNA or nucleotides are probably due to their being broken down to individual ribonucleosides by phosphatases. For example, injection of cytoplasmic RNA from the rat liver into mice during chronic poisoning with CCl.sub.4 reduced the mortality among the animals. Moreover, the number of foci of necrosis were reduced and the number of interlobular connective tissue fibers in the liver were increased. An increase in the mitotic activity of the liver cells was also observed. Chernukh, A. M., et al., Bull. Exp. Biol. Med. 70:1112-1114 (1970).
Administration of RNA, mixed nucleotides, or hydrocortisone, either alone or in various combinations, was found to increase tyrosine-alpha-ketoglutarate activity in rat liver. Administration of RNA or nucleotides elevated enzymatic activity beyond the level attained after hydrocortisone administration alone. The authors speculated that the RNA or nucleotides may act via two mechanisms: first, a nonspecific stress effect, mediated through stimulation of adrenal steroid release, or secondly, through provision of limiting substrates for RNA synthesis. Diamondstone, T. I., et al., Biochim. Biophys. Acta 57:583-587 (1962). In a study on human patients with hepatic cirrhosis, administration of cytidine and uridine improved insulin sensitivity in the cirrhotic patient, but had no effect on insulin sensitivity in patients without liver disease. Ehrlich, H., et al., Metabolism 11:46-55 (1962).
In a study of repair after mechanical trauma in the liver, a rapid, sustained increase in RNA content of cells at the border of experimentally induced trauma was observed. DNA concentrations in the traumatized area began to rise on the third day after injury and continued to rise till the 10th day. The diabetic rat liver, in contrast, showed poor RNA and DNA contents. Increases in the tissue content of RNA and DNA around the traumatized site were delayed and strongly depressed relative to nondiabetic livers. The failure of RNA synthesis, which gives rise to poor wound healing in the diabetic liver, was attributed to deficient activity of the hexose monophosphate pathway of glucose metabolism as observed in diabetics. Shah, R. V., et al., J. Anim. Morphol. Physiol. 25:193-200 (1978); Shah, R. V., et al., J. Anim. Morphol. Physiol. 21:132-139 (1974).
In another study, the availability of UDPG was found to be rate-limiting for hepatic glycogen synthesis under some conditions. When cultured hepatocytes were incubated with uridine, there was an increase in the incorporation of glycose into glycogen and tissue uridine nucleotide pools were expanded. When uridine was omitted from the incubation mixture, levels of UTP and UDPG dropped markedly during a 1 hour incubation. Songu, E., et al., Metabolism 30:119-122 (1981). In a study of patients with alcoholic hepatitis, a beneficial effect of uridine-diphosphoglucose, when administered intramuscularly or intravenously, was found in biochemical indices as well as physiological and psychological symptoms. Thus, pyrimidine nucleosides are effective in treatment of some forms of pathology of the liver.
(4) Diabetes
Nucleosides are also useful for the treatment of diabetes. In experimental diabetes, RNA synthesis is reduced in a number of tissues. Administration of oral sodium ribonucleate was found to increase the rate of RNA biosynthesis in tissues of diabetic rats. Germanyuk, Y. L., et al., Farmakol. Toksikol. 50-52 (1979). This effect is probably a result of hydrolysis of the administered RNA to give individual ribonucleotides and/or ribonucleosides. The failure of RNA synthesis in the diabetic rat liver has been attributed to the deficient activity of the hexose monophosphate pathway of glucose metabolism in diabetes. Shah, R. V., et al., J. Anim. Morphol. Physiol. 25:193-200 (1978).
(5) Phospholipid Biosynthesis
Cytidine nucleotides have been implicated in phospholipid biosynthesis. For example, Trovarelli, G., et al., Neurochemical Research 9:73-79 (1984), disclose that upon the intraventricular administration of cytidine into the brain of rats, a measurable increase in the concentrations of all the nucleotides, CDP-choline, CDP-ethanolamine, and CMP occurred. The authors state that the low concentration of free cytidine nucleotides in nervous tissue likely limits the rate of phospholipid biosynthesis.
(6) Brain
Administration of cytidine and uridine has also been reported to be effective in the treatment of various neurological conditions in animals. For example, Dwivedi et al., Toxicol. Appl. Pharmacol. 31:452 (1978) disclose that uridine, administered by intraperitoneal injection in mice, is an effective anticonvulsant, providing strong protection against experimentally-induced seizures.
Geiger et al., J. Neurochem 1:93 (1956) disclose that the functional condition of circulation-isolated cat brains perfused with washed bovine erythrocytes suspended in physiological saline remained normal for only about 1 hour. If either the animal's liver was included in the perfusion circuit, or cytidine and uridine were added to the perfusate, the functional condition of the brain remained good for at least 4 to 5 hours. The cytidine and uridine tended to normalize cerebral carbohydrate and phospholipid metabolism. The authors suggest that the brain is dependent upon a steady supply of cytidine and uridine, which are perhaps normally supplied by the liver.
Sep.epsilon., Minerva Medica 61:5934 (1970), disclose the effect of daily intramuscular injections of cytidine and uridine in neurological patients, most suffering from cerebrovascular disorders. Beneficial results were obtained, particularly with respect to restoration of motor function, and in improving recovery after cranial trauma. No undesirable side effects were observed.
Jann et al., Minerva Medica 60:2092 (1969) disclose a study of patients with a variety of neurological disorders which were treated daily with intramuscular injections of cytidine and uridine. Beneficial effects were observed, particularly in cerebrovascular disorders involving motor function and mental efficiency. No undesirable side effects were observed.
Monticone et al., Minerva Medica 57:4348 (1966), disclose a study of patients with a variety of encephalopathies which were treated with daily intramuscular injections of cytidine and uridine. Beneficial effects were found in most patients, particularly those with cerebrovascular disorders or multiple sclerosis. No undesirable side effects were observed.
One method that has heretofore been used, in effect, to introduce cytidine equivalents into patients is the administration of cytidine-diphosphocholine (CDP-choline). Cytidine-diphosphocholine, an intermediate in the biosynthesis of phosphatidyl choline (lecithin) is used therapeutically in Europe and Japan (under such names as Somazina, Nicholin, and Citicholine) for treating a variety of disorders. Therapeutic efficacy has been documented in central nervous system pathologies including brain edema, cranial trauma, cerebral ischemia, chronic cerebrovascular diseases, and Parkinson's disease. The mechanism underlying the pharmacological actions of this compound is believed to involve support of phospholipid synthesis, restoration of the biochemical "energy charge" of the brain, or a possible effect on neurotransmitter (particularly dopamine) function.
Examination of the fate of CDP-choline following its administration to animals or humans indicates that this compound is very rapidly degraded, yielding cytidine, choline, and phosphate. After oral administration, no intact CDP-choline enters the circulation, although plasma cytidine and choline concentrations rise. After intravenous injection, breakdown to cytidine and choline occurs within about 30 seconds. Therefore, it is difficult to attribute the therapeutic effects of exogenous CDP-choline to the entry of this compound directly into cellular metabolism.
Therapeutic benefits in cerebral pathologies similar to those obtained with CDP-choline have been achieved following administration of cytidine and uridine to humans and experimental animals. Therefore, CDP-choline appears to serve merely as an inefficient, expensive "prodrug" for cytidine, use of which perhaps hinders rather than enhances the transport of cytidine to target tissues, compared to administration of cytidine itself. Administration of choline by itself does not result in the therapeutic benefits obtained after administration of either cytidine or CDP-choline. It would thus be advantageous to develop methods for delivering cytidine to the brain that are less expensive and/or more efficient than administration of CDP-choline or cytidine itself.
Uridine-diphosphoglucose, uridine-diphosphoglucuronic acid, and uridine diphosphate also have been shown to improve certain aspects of liver function. Since such phosphorylated compounds, as well as CDP-choline, must in general be dephosphorylated before they will enter cells, administration of uridine, or derivatives of uridine, should represent a substantial improvement, in terms of both efficiency and cost, over the use of the phosphorylated pyrimidine derivatives.
(7) Immunological System
Cytidine and uridine may also have important influences on the function of the immune system. Kochergina et al. (Immunologiya 0(5):34-37, 1986) disclose that administration of either cytidine-5'-monophosphate or uridine-5'-monophosphate to mice simultaneously with an antigen (sheep red blood cells) results in a strong enhancement (relative to the response in animals treated with only the antigen) of the humoral immune response to a subsequent challenge with the antigen. Enhanced responsiveness of T-helper lymphocytes was reported to underlie this phenomenon. Thus, cytidine or uridine may be useful as adjuncts to improve the efficacy of vaccines, to improve the responsiveness of the immunce system in an immunocompromised patient, or to modify immune response in experimental animals. Van Buren et al., (Transplantation 40:694-697 (1985)) disclose that dietary nucleotides are necessary for normal T-lymphocyte function; they did not, however, evaluate the influence of supra-normal amounts of dietary or parenterally administered nucleotides or nucleosides.
In vivo, exogenous uridine itself is catabolized to a large extent, rather than taken up and utilized for nucleotide synthesis. Gasser, T., et al., Science 213:777-778 (1981), disclose that the isolated, perfused rat liver degrades more than 90% of infused uridine in a single passage. Much of the uridine released by the liver in the portal vein is from degradation of liver nucleotides synthesized de novo rather than from arterial uridine. This accounts for the poor utilization of administered uridine in peripheral tissues.
For example, Klubes, P., et al., Cancer Chemother. Pharmacol. 17:236-240 (1986), disclose that after oral administration of 350 (mg/kg of uridine in mice, plasma levels of uridine were not perturbed. In contrast, plasma levels or uracil, a catabolite of uridine, peaked at 50 micro then declined and returned to normal after 4 h. Elevation of plasma uridine levels was observed only after oral administration of high doses of uridine (3500 mg/kg). However, such doses would be much too high for an adult human since they would amount to about 200 g/dose.
A novel strategy for improving the bioavailability of cytidine or uridine after oral or parenteral administration is to administer derivatives of cytidine or uridine containing particular substituents which improve the pharmacokinetic or other pharmaceutical properties (e.g., transport across biological membranes) of these nucleosides. Properly chosen substituents, of which acyl substituents are best) will undergo enzymatic or chemical conversion back into cytidine or uridine following administration.
Certain acylated uridine and cytidine derivatives are known, per se. Honjo, et al., in British Patent No. 1,297,398, describe N.sup.4,O.sup.2',O.sup.3',O.sup.5' -tetraacylcytidines and a process for their preparation. The acyl substituents are those derived from fatty acids having from three to eighteen carbon atoms.
Beranek, et al., Collection Czechslovak Chem. Commun. (vol. 42, 1977), p. 366-369, describe the preparation of 2',3',5'-tri-O-acetylcytidine hydrochloride from cytidine by reaction with acetyl chloride in acetic acid.
Sasaki, et al., Chem. Pharm. Bull. (vol. 15, 1967), describe the acetylation of cytidine with acetic anhydride to form N.sup.4 -acetylcytidine, 5'-O-acetylcytidine and N.sup.4,5'-O-diacetylcytidine, among other compounds.
U.S. Pat. No. 4,022,963 to Deutsch, describes methods for acetylating all of the hydroxyl groups in the sugar portion of some nucleosides which include uridine, by a process including the addition of excess acetic anhydride.
Samoileva, et al., Bull. Acad. Sci. USSR Div. Chem. Sci. Vol. 30, 1981, p. 1306-1310, disclose a method for synthesizing aminoacyl or peptidyl derivatives of cytidine or cytidine monophosphate using insoluble polymeric N-hydroxysuccinimide. N.sup.4 -BOC-alanyl cytidine was prepared. The aminoacyl derivatives of cytidine were synthesized as probes for studying the function of nucleases.
Japanese Patent Publications Nos. 51019779 and 81035196 assigned to Asahi Chemical Ind. KK describe methods for preparing N.sup.4 -acyl-cytidines by reacting cytidine with acid anhydrides derived from fatty acids containing 5 to 46 carbon atoms. The products are said to be lipophilic ultra-violet absorbing agents and are also useful as starting compounds in the preparation of anti-tumor agents.
Watanabe, et al., Angew. Chem., Vol. 78, 1986, p. 589 describe methods for selective acylation of the N.sup.4 -amino group of cytidine wherein methanol is used as a solvent and acid anhydride as acylating agent. Compounds prepared were N.sup.4 -acetyl-, N.sup.4 -benzoyl-, and N.sup.4 -butyryl-cytidine.
Rees, et al., Tetrahedron Letters, Vol. 29, 1965, p. 2459-2465 disclose methods for selective acylation of the 2 position on the ribose moiety of ribonucleosides. Uridine derivatives were prepared including 2'-O-acetyluridine, 2'-O-benzyluridine, and 2',5'-di-O-acetyluridine and other derivatives. The compounds were prepared as intermediates in oligo-ribonucleotide synthesis.